Testosterone stimulated the growth of song control nuclei in female European robins. Photomicrographs are shown for the HVC (A-C, E-G) and the RA (D, H) after staining with 0.1 % thionin (Nissl staining: A, D, E, H) and an anti-NeuN antibody (B, C, F, G), respectively. Sagittal forebrain cryostat sections were obtained from control (left panels) and testosterone (right panels) female robins. Arrows indicate the border of a song control nucleus; scale bars represent 100 μm (C for C and G) and 500 μm (B for B and F; D for D and H), respectively. Note that the testosterone treatment increased the volume (A, B vs E, F; D vs H), neuron size and spacing (C vs G) of a song control nucleus.